1. Field of the Invention
The present invention relates to a wireless communication system. More particularly, the present invention relates to a method for processing a Channel State Information Reference Signal (CSI-RS) in a wireless communication system based on a multiple access scheme.
2. Description of the Related Art
In 3rd Generation (3G) advanced wireless mobile communication system standards, two types of reference signals are specified, namely a Common Reference Signal (CRS) and a Dedicated Reference Signal (DRS). CRS is referred to as either a cell-specific RS or Common RS (CRS) in a 3G Partnership Project (3GPP) Long Term Evolution (LTE) standard and is monitored by all User Equipments (UEs) in a cell of a corresponding base station. For multiple antenna transmission, reference signal patterns are defined to distinguish between the antenna ports for channel estimation and measurement. In an LTE system, a maximum of 4 antenna ports can be supported. DRS denotes a reference signal that is separately transmitted from a CRS and listened to by a UE indicated by the base station. In a 3GPP LTE-Advanced (LTE-A) system, this reference signal is referred to as a UE-specific RS, a DRS, or a Demodulation Reference Signal (DMRS) and is used for supporting a data traffic channel transmission with non-codebook based precoding at the base station.
In the LTE-A system, which is an advanced form of the LTE system, a DeModulation Reference Signal (DM-RS) is transmitted for supporting channel estimation with 8 layers in addition to the aforementioned CRS and DRS.
FIG. 1 is diagram illustrating configurations of a radio frame, a subframe, and a Physical Resource Block (PRB) for transmitting CRS in an LTE system according to the related art.
Referring to FIG. 1, a radio frame is divided into 10 subframes, each having a length of 1 msec. This means that a radio frame has a length of 10 msec and consists of 10 subframes as shown in FIG. 1. In FIG. 1, reference number 110 denotes one of the subframes constituting the radio frame. For each subframe, an evolved Node B (eNB) performs transmission over the system bandwidth in Orthogonal Frequency Division Multiple Access (OFDMA). One subframe consists of a plurality of Physical Resource Blocks (PRBs). One PRB consists of 12 subcarriers. For one subframe, the subcarriers are arranged at a regular interval in the frequency domain. In FIG. 1, reference number 120 denotes one of the PRBs constituting the system bandwidth. In the LTE signal structure of FIG. 1, a number of PRBs is determined depending on the system bandwidth.
The PRB 120 is a time-frequency resource region as denoted by reference number 130. As denoted by reference number 130 of FIG. 1, each PRB is a time-frequency resource region consisting of 12 subcarriers in the frequency domain and 14 OFDMA symbol durations in the time domain. The resource unit defined by one subcarrier and one OFDM symbol duration is referred to as a Resource Element (RE), and one RE can carry one data symbol or reference signal symbol.
The PRB 130 consists of 12 subcarriers and 14 OFDM symbol durations. This means that a PRB 130 consists of a total of 168 REs. The first three OFDM symbol durations of the PRB 130 are assigned as a control region in which the eNB uses a control channel for transmitting control information with which the UE can receive a traffic channel. Although the control region is defined by the first three OFDM symbol durations, it can be configured with the first one or two OFDM symbol durations depending on the eNB's determination.
In FIG. 1, reference number 140 denotes a data RE for use in transmitting traffic channel. Reference number 150 denotes a CRS RE for use in transmitting a CRS for a UE's channel estimation and measurement. Since the positions of the data RE and CRS RE are known to the eNB and UE, the UE can receive the CRS and traffic channel correctly in the PRB. Unless specifically stated otherwise, all indexing starts from 0 in the following description. For example, in FIG. 1, the 14 OFDM symbols constituting the PRB are indexed from 0 to 13.
FIG. 2 is a diagram illustrating resources allocated for a UE to report a channel quality measurement to an eNB in an LTE system according to the related art.
Referring to FIG. 2, the UE measures a channel quality of all the PRBs within the system band for the subframe 230 including a plurality of PRBs. In order to measure the channel quality in each PRB, the UE uses the CRS 220 transmitted by the eNB. Since the CRS is transmitted at the same transmission power in all of the PRBs, the UE can determine which PRB has relatively higher channel quality by comparing the received signal strengths of the CRSs received in respective PRBs. Also, it is possible to determine the data rate which each PRB can support depending on the absolute received signal strength. The channel quality information is mapped in the form of channel feedback information and then reported to the eNB using the uplink control channel as denoted by reference number 240 of FIG. 2. Based on the channel feedback information transmitted by the UE, the eNB performs downlink transmission in the subframes 251, 252, 253, 254, and 255. The eNB can acquire the information on the data rate, preferable precoding, and preferable PRB supported by the UE based on the channel feedback information transmitted by the UE and performs downlink scheduling and Adoptive Modulation and Coding (AMC) based on the acquired information.
In FIG. 2, the eNB uses the channel feedback information 240 before the receipt of the next channel feedback information 260. Although it is depicted that only one UE transmits the channel feedback information in FIG. 2, a real world system is typically designed such that a plurality of UEs can transmit the channel feedback information simultaneously.
However, the method described above has a number of problems. For example, in the LTE system, the UEs measure the channel quality based on the CRS transmitted by the eNB. In case of measuring the channel quality with a CRS as shown in FIG. 2, the number of layers for the eNB to transmit with Multiple-Input Multiple-Output (MIMO) technology is limited by the number of antenna ports of a CRS. According to the standard, the LTE system can support up to 4 antenna ports. Since more than four CRS antenna ports are not supported, the MIMO transmission of the eNB is limited to a maximum of four layers.
Another problem with CRS-based channel estimation and measurement of the UEs is that the eNB must always transmit a CRS. Accordingly, in order to support more than four antenna ports, an additional CRS should be transmitted. This means that the limited radio resource is excessively concentrated on transmitting a CRS for channel estimation measurement, resulting in bandwidth inefficiency.